Hello, I will provide you feedback but know that is generally frowned upon to use this platform where people come with real frustrations and hurdles and use it for advertisement.
You website has a lot of flaw, granted is a landing page, and it could be greatly improved. Quora is great not only because they have the work to gather the community of experts and non experts on an open platform where questions, surveys,etc can be shared by individuals and companies alike. They provide their features for free and requires no log in to view what is going on. Yours seems to a landing asking for contact information but you failed to provide examples or video or anything that explains easily in 1-3 seconds what it is that you are and why you want my email and how would I benefit from giving it to you.

With that said, the idea of providing a niche for PHD students is cool and I can see that needed, as we go through higher education our tutoring resources become less and less available. I would however try to be everything to everyone - focus on one core value to one core group of people - reason: You don't have the time and money to expose and cater to everyone.

Consider doing research in other websites with similar services like chegg.com

Also, there is a class I took through onemnth here http://mbsy.co/onemonth/18573440 - it shows you how to build landing pages more effectively.

best of luck :)
Humberto Valle